Client 1 -> 192.168.108.0/24 and 10.8.0.4
openvpn server -> 10.8.0.0/24 and 10.10.40.19
client 2 -> 192.168.100.50 and 10.8.0.10

How to make client 2 network accessible from client 1 without changing client 2 router details

  1. First we need to be sure that IP forwarding is enabled on client 2 machine. Very often this is disabled by default. This is done by running the following command line as root:
sudo sysctl -w net.ipv4.ip_forward=1

This change is temporary so to make it permanent /etc/sysctl.conf In this file you should have a line stating:

net.ipv4.ip_forward = 1

  1. Add push "route 192.168.100.0 255.255.255.0" to openvpn server config. and iroute 192.168.100.0 255.255.255.0 to client file in ccd folder

  2. Add iptables rule to client 2 machine.

sudo iptables -t nat -I POSTROUTING -d 192.168.100.0/24 -j MASQUERADE

Other useful iptables command for debugging

sudo tcpdump -i tun0 icmp
sudo iptables -t nat -L --line-numbers
sudo iptables -t nat -D POSTROUTING 1

Install different version of Openvpn

wget https://swupdate.openvpn.org/community/releases/openvpn-2.5.7.tar.gz
tar xfz openvpn-2.5.7.tar.gz
sudo apt install libssl-dev liblzo2-dev libpam0g-dev
cd openvpn-2.5.7
./configure
sudo make
sudo make install